MECHANOSENSITIVITY OF AN EPITHELIAL NA- RELEASE FROM CA2+ BLOCK( CHANNEL IN PLANAR LIPID BILAYERS )

A family of novel epithelial Na+ channels (ENaCs) have recently been c loned from several different tissues. Three homologous subunits (alpha , beta, gamma-ENaCs) form the core conductive unit of Na+-selective, a miloride-sensitive channels that are found in epithelia. We here repor t the results of a study assessing the regulation of alpha, beta, gamm a-rENaC by Ca2+ in planar lipid bilayers. Buffering of the bilayer bat hing solutions to [Ca2+] < 1 nM increased single-channel open probabil ity by fivefold. Further investigation of this phenomenon revealed tha t Ca2+ ions produced a voltage-dependent block, affecting open probabi lity but not the unitary conductance of ENaC. Imposing a hydrostatic p ressure gradient across bilayers containing alpha,beta,gamma-rENaC mar kedly reduced the sensitivity of these channels to inhibition by [Ca2]. Conversely, in the nominal absence of Ca2+, the channels lost their sensitivity to mechanical stimulation. These results suggest that the previously observed mechanical activation of ENaCs reflects a release of the channels from block by Ca2+.